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Disclaimer: The notes below were produced by the author for the purposes of preparing for an Officer-of-the-Watch oral examination in November 2015. As such they are intended only as simplified summaries and aide memoires and therefore do not give a true reflection of the original content or intention of the documents to which they refer. These notes are likely to contain inaccuracies and omissions that make them unsuitable as an alternative to a proper study and understanding of the current revision of the original document(s). They are posted here as a study aid and may be used freely for non-commercial purposes, however any such use remains the responsibility of the reader. Where a copyright claim is made the domain holder should be contacted in the first instance to have the content removed.
Electronic Chart Display and Information System (ECDIS)
To ensure standardisation International Hydrographic Office issued Special Publications:
S52 with Presentation Library as Appendix
IHO document determining the specifications for chart content and display aspects of ECDIS. To ensure the colours and symbols on an ECDIS screen are unambiguous and that once familiar seafarers can move between ships and systems without confusion. Includes things like text size, opacity of a colour indicating a feature, similarity of symbols to paper charts.
S57
Defines the transfer standard to be used for data transfer between national hydrographic offices and to their customers to ensure that none of the data’s meaning is lost. Essentially this is the language your information must be expressed in for the listening computer to grasp its full meaning.
S63
Describes the recommended standard for the protection of ENC information. This includes encrypting ENC information in order to:
Prevent unauthorised access
To restrict customer access to those cells for which licenses are held
To ensure that only information from approved sources can be accessed
The Maritime and Coastguard Agency issued MIN 503(replacing MIN 442 and 445) which clarifies training requirements for UK officers who have ECDIS as their primary means of navigation. It requires generic training (IAW STCW I/14 – 5) and familiarisation with the specific type of ECDIS used on the officer’s ship (IAW ISM 6.3 & 6.5).
SOLAS Chapter V Regulation 19 (carriage requirements for shipborne navigational systems and equipment) now includes a schedule for vessels on international voyages to start carrying ECDIS (19.2.10). All vessels must have an ECDIS by July 2018.
Type Approval of ECDIS is performed by a Notified Body (classification society) on behalf of the Flag State who then issue the manufacturer with a Type Approval Certificate to issue with the ECDIS to purchasing customers. Type Approval is mandated by SOLAS Chapter V Regulation 18.
If ECDIS is the primary means of navigation this must be indicated on the Record of Equipment that is attached to the vessel’s Safety Certificate. The ship’s SMS must cover relevant requirements and procedures to do with ECDIS. An independent backup of either paper charts or another independent ECDIS system must also be in place and indicated on the document. If the backup is another ECDIS any changes to either must be immediately copied to the other so that if one becomes inoperable the other is immediately ready to be used. A third ECDIS unit is often installed for planning purposes. This one doesn’t push out changes immediately so that the Navigator can use it for planning but usually offers this function so that if an ECDIS goes down the ship can still sail with this as the backup.
ECDIS must have Uninterrupted Power Supply (UPS) backup so the system doesn’t need to reset on loss of main power. UPS must therefore be AC rather than a simple battery backup.
Minimum number of sensor inputs to ECDIS is three – position, speed log, heading (eg GPS, Doppler log, Gyro Compass)
Electronic Navigational Chart(ENCs),also known as Vector Charts, are the primary type of chart for use with ECDIS. An ENC is a database of information that can be displayed in different ways. This allows the user to select different layers of information, zoom in and out without distortion or reduction of picture quality or a change in font size
As the user can select the layers of information from an ENC that are displayed ECDIS must offer three levels of detail – Base, Standard and Custom
Base includes the most important information but isn’t legal for navigation
Standard is the default and legally compliant level, it includes:
Custom is user-defined
There is also a proposal being considered to include a Safe Mode that would allow the system to be quickly set up so that the information displayed on screen would be safe for navigation on that vessel. Essentially this would be Standard Mode but taking into account your vessel’s current draft to display safe contours
Contours
Safety – draft + squat + UKC – HoT (defines the border between safe and unsafe water)
Shallow – draft + squat – HoT (useful to show the gradient of the bottom beyond the safety contour)
Deep – the depth at which squat and interaction first take effect
The ECDIS doesn’t draw a contour for the exact depth you choose but selects from what is available on that ENC. For safety reasons it chooses the most appropriate deepercontour (ie for a 20m contour you need to ask for 19.9m or it might display 25m or more depending on what options are included in the chart)
Scale Minimum (SCAMIN)
SCAMIN values are assigned to chart features and determine at which user selected scale or range a feature would stop being displayed. SCAMIN aims to prevent too much ‘clutter’ that would render the display useless when a larger range is selected (like a view of the UK with all buoys displayed at once). This presents a danger in that the user can select a scale different from the one the chart was designed for and not see navigationally significant features. An alarm will highlight when this is the case.
Use of Raster Navigational Charts (RNCs) with ECDIS
As with paper charts you must have the best scale ENC charts available for your whole voyage. In some areas no ENCs are available so an RNC on ECDIS can be used but you must:
-Have a paper copy of relevant charts
-Perform a risk assessment (IAW MGN 285) identifying all hazards with particular reference to your vessel and put appropriate control measures in place
-Set the ECDIS in RCDS mode
-Create a layer highlighting hazards so the anti-grounding cone will still alarm
-Emphasis the use of traditional techniques such as parallel indexing
Radar Interface Overlay (RIO)
This allows your ECDIS to display the current radar picture over your chart. You can use this to see if the land picked up by your radar is where the chart says it should be based on your position. Every ECDIS must offer a Radar or ARPA overlay
Potential Problems with ECDIS
Overscale – the user is looking at the chart on the wrong scale and features are being omitted due to their SCAMIN values
Power Failure – if the ECDIS loses main power your UPS should kick in, if it doesn’t or the failure is within the unit you will need to use your backup ECDIS or paper charts
Wrong Choice of Layers – important features are not being displayed because the Standard display is not selected
Input Failures – your GPS, Log or Compass is giving the ECDIS false information
Over-reliance – the OOW can be tempted to navigate using the ECDIS screen rather than the windows and radars
Chart Error – the charts are not correctly updated, particularly with Navigational Warnings that are sent out via GMDSS and the IHO website
For safety purposes ECDIS is required to draw the user’s attention to certain events and conditions. For some an audible alarm is required, while others only require a visual indication as follows:
AlarmCrossing safety contour
Alarm or IndicationArea with special conditions
AlarmDeviation from route
AlarmPositioning system failure
AlarmApproach to critical point
AlarmDifferent geodetic datum
Alarm or IndicationMalfunction of ECDIS
IndicationDefault safety contour
IndicationInformation overscale
IndicationLarger scale ENC available
IndicationDifferent reference system
IndicationNo ENC available
IndicationCustomized display
IndicationRoute planning across safety contour
IndicationRoute planning across specified area
IndicationCrossing a danger in route
IndicationMonitoring mode
IndicationSystem test failure
Radio Detection and Ranging (RADAR)
X-Band Radar broadcasts on 9.2-9.4GHz, S-Band 3-3.1GHz, each unit is on a slightly different frequency to avoid interference caused by apparent return signals originating from another ship’s radar. When this happens it appears as curving spokes on the radar screen.
Switching on the Radar:
For safety reasons you must check no one is working next to the scanners before they start turning and transmitting. This should be a visual check and a check on any permits for working at height that might indicate personnel nearby
Ensure that inputs are available from your compass and speed log so that the radar can indicate heading and calculate relative vectors of contacts
Start the radar observing the warm-up time
Set Gain, Sea Clutter, Rain Clutter and Tuning
Changing the tuning of the radar will adjust the frequency of the transmission. Manual should be performed if possible as the auto function just maintains the setting. To tune try to maximise the bar or look at distant contacts to see if you can improve the return.
Potential Radar Errors:
Blind Sectors – where you get no return due to intervening ship’s structure. Cannot be within a sector from right ahead to 22.5° abaft the beam either side. Sectors cannot exceed 5° and are considered to be a single area if not separated by at least 3°.
Shadow Sectors – these are similar in principle to Blind Sectors but are caused by objects not on the ship
Multiple Echoes – a strong return from a nearby target bounces off your own ship and the contact a second time resulting in a second contact at an equal distance beyond the real one. It will appear larger as the beam is 1° wide meaning greater width at greater range.
Side Lobe Error – the width of the radar beam itself and energy escaping in undesired directions means that close contacts might appear stretched. Dots will become arcs as they get closer because the contact is increasingly detected by the transmitter/receiver when it isn’t pointed straight at it. This is why a SART goes from dots to rings. The effect can be reduced by reducing range scale and therefore transmission power.
Heading Marker Error – the heading of your ship and the bearings of contacts are not being displayed correctly because the input or the system are faulty. The radar system manual will tell you how to adjust this manually
Scintillation Error (Glint) – stationary objects appear to be moving because you detect a different part of them as their aspect changes. Rarely happens
Backlash Error – the gears turning the radar are worn allowing the wind to sway the transmitter/receiver causing the radar to point down a slightly different bearing than intended
Radar Range Discrimination
The pulse length selected will determine how far apart in a direction extending away from your vessel contacts need to be to appear separate. This is usually a minimum of 30m or 1% of the range scale in use. Shorter pulse lengths gives greater discrimination but longer pulse lengths give a stronger return. Radars have a default pulse length for each range scale but the user can usually select short, medium or long pulse options suitable for each to try and improve the picture
Radar Bearing Discrimination
The radar beam is 1° wide meaning that contacts are detected at the leading and trailing edge of this 1° cone. Therefore contacts appear to be at least 2° wide on the radar. The radar only distinguishes in ½° increments so two contacts need to be at least 2½° apart to be displayed as separate contacts (realistically 3°). This is what is meant by the requirement for type-approved radar to have 2½° discrimination
Radar Ranges and Bearings
These can be used for position fixing but the ranges are more reliable than the bearings due to the discrimination limitations (3°) described above
Parallel Index Lines
Are a tool for position monitoring as they will indicate the distance of detected land and fixed navigational marks off your heading and therefore how closely you are following your intended track
Rain Clutter
This desensitises the radar receiver to the initial returns from all targets. Consequently Gain should be increased so that the remaining returns from real contacts are emphasised on the screen
Radar Modes
True Motion – like looking down on a circle of sea from above, your ship will move across the screen until it comes towards the edge and then snap back to the other side again. This can be good for coastal navigation and restricted visibility
Relative Motion – your ship remains in the centre and everything slides by it
True Vectors – the projected true motion of your vessel and other contacts is displayed on the screen as a line extending from them. The downside of this mode is that it isn’t immediately obvious if you are on a collision course with another vessel, you have to look where both of your vector lines are pointing
Relative Vectors – your ship has no vector, all other contacts have vectors relative to you. You can see immediately whether a contact is on a collision course because its vector is pointed at you in the centre of the screen
Sea stabilised – the radar uses your log speed (STW) to factor out the movement of the water’s surface from all vectors. This is important for anti-collision work as drifting objects will appear stationary but fixed objects will appear to be moving as they resist the current and you don’t
Ground stabilised – the radar uses your GPS speed (SOG) and shows drifting objects as moving and fixed objects like land as stationary
Raconsare semi-active beacons that detect your radar pulse and send a series of pulses back spelling out a morse character that appears on the screen extending beyond them making them more useful as navigational marks or warnings
Global Positioning System (GPS)
Designed to use 24 satellites but there are spares. A large number are needed to provide full coverage as the satellites orbit the Earth
Each satellite transmits a Pseudorandom Code, Almanac Data and Ephemeris Data. The Ephemeris Data is for the ground stations telling them how the satellite’s doing (orbit, clock accuracy etc). The Almanac Data tells the user where all the satellites should be at any given moment. The Pseudorandom Code identifies the satellite (because it’s not really random) and has timing marks to allow your system to compare the arrival time with the transmit time giving you a distance from that satellite.
GPS determines position by:
GPS satellites send out a signal with a time code so the ground unit can measure how long it took to reach them and thus the range of the satellite
The range from a satellite describes a sphere around it
Ranges from two satellites would give two overlapping spheres describing a circular plane of possible locations
Adding a range from a third would cut the plane at two points giving a 2D fix. From this we would probably have a position because one of the two points will be in space
Adding a fourth range would reduce the possible position to one location – a 3D fix
Potential GPS Errors:
Timing Error – the GPS unit doesn’t have an atomic clock of its own so it misjudges the distance of satellites slightly causing the position lines to meet in a cocked hat. Since it misjudges the distance of each satellite by the same amount it can use a mathematical process called Trilateration to determine what correction value causes the lines to resolve in a good position fix. This is done automatically.
Atmospheric Error – daily differences in atmospheric pressure and the angle a signal takes through the atmosphere causes different delays. An average correction is applied and small errors remain, contributing to the imperfect accuracy of GPS.
Almanac Error – satellite is not exactly where the almanac data projected. The ground stations will either correct the Almanac Data or the satellite’s orbit within a 6-12 hours.
Multi-path Error – signal comes to the ship after bouncing off an obstruction extending its travel time and adding to the apparent range.